Servomotor control apparatus

ABSTRACT

The invention is a servomotor control apparatus for controlling the positioning of a movable element of an industrial robot or NC machine tool. An NC unit or a robot controller detects displacement from a movable element that is to be positioned, such as a table, forms a position command regarding a servomechanism, and has a correction circuit (A) which receives quadrant data (BLF) regarding a backlash correction command. When there is a quadrant reversal for each axis of the movable element, frictional torque corresponding to the axis is stored and a torque correction command (FR) corresponding thereto is outputted. A torque command of a fully-closed loop servo system for performing control based on a fed back position detection signal is corrected by the torque correction command. The servomotor control apparatus can be applied to semi-closed loop servo system in which a backlash correction is possible, and not just to a fully-closed loop servo system.

FIELD OF THE INVENTION

This invention relates to a servomotor control apparatus for controllingthe positioning of a movable element of an industrial robot or NCmachine tool.

DESCRIPTION OF RELATED ART

A fully-closed loop-type control circuit is illustrated in FIG. 3 as anexample of a CNC servo-control circuit. In the Figure, an informationprocessing circuit X includes a CPU and a memory and forms a commandsignal for a servomechanism Y. The servomechanism Y is constituted by acomparator circuit a for comparing the command signal and a fed backposition signal, a servo drive circuit b to which are inputted an errorsignal obtained by the comparator circuit a and a fed back velocitysignal, a servomotor c, a velocity detector d for detecting the velocityof the servomotor c, and a linear scale e for position detection mountedon a mechanical load f such as a table.

The characterizing feature of a fully-closed loop servo system is thatan arrangement in which the machine body is incorporated in thepositioning servo loop is realized by mounting the position detector onthe mechanical load f of the servomotor.

FIG. 4 is a block diagram of a servo system of this type, in which Kdenotes position gain, k₁ and k₂ gains, T_(L) disturbance, K_(T) atorque constant, J_(m) rotor inertia, and J_(L) load intertia.

The relationship between the position command and torque command of theservo system shown in FIG. 4 can be expressed by representing anintegration term (Z⁻¹) by a transfer element subjected to a Z conversion(a pulse transfer function conversion), as shown in FIG. 5. In FIG. 5,MC(i) indicates a move command formed by an NC apparatus, VEL(i)indicates a servomotor velocity signal, and POS(i) represents an amountof movement of the machine in a sampling period T.

Besides using this fully-closed loop system, it is also possible tocarry out positioning control by a semi-closed loop-type servo-controlsystem which extracts a position signal from a servomotor output shaftin front of the mechanical load or from a ball screw shaft coupled tothe servomotor output shaft. With this system, a comparativelyinexpensive resolver, pulse encoder or the like is used as a positiondetecting element and it is possible to realize dynamic precision.However, since a mechanism-related error such as feed screw pitch errorhas a direct influence upon control precision, static precision withregard to position is poor in comparison to dynamic precision. The typeof, servo-control system that is used is determined by the userdepending upon the type of position detecting element employed and towhat degree of accuracy the mechanical load is to be controlled.However, there are many cases in which an NC apparatus will make commonuse of both systems.

When the rotating direction of a servomotor changes in response to acommand for changing the direction of movement of a table or arm, aresponse delay regarding the position command occurs in accordance withthe servomotor velocity-torque characteristic shown in FIG. 2. Responsedelays occurs because each axis of the mechanical load possesses acontrol error element ascribable to backlash or friction. In otherwords, the rotation of the servomotor of a predetermined axis approachesa stopped state when there is a reversal in the direction of rotation,and torque is no longer proportional to velocity before and after theoperating quadrant reverses.

With the semi-closed loop servo-control system, a backlash command fromthe NC apparatus side is inputted to the servomechanism together withthe position command, thereby correcting the servomotor position signalto improve upon the response delay when the direction of rotationreverses. However, as described in connection with FIG. 3, thefully-closed loop servo-control system is arranged so that themechanical load is included in the positioning servo loop, and abacklash component is contained in the position signal obtained from thelinear scale e. As a result, the servo-control system position signalcannot be corrected by the backlash correction outputted from the NCapparatus side. In consequence, the backlash correction command cannotbe used commonly by the same NC apparatus in both the semi-closed loopservo-control system and fully-closed loop servo-control system.Accordingly, there is a decline in the utilization efficiency of the NCapparatus and the response delay at the time of the reversal indirection cannot be ameliorated with regard to the machine in thefully-closed loop servo-control system.

SUMMARY OF THE INVENTION

The present invention has been devised in order to solve theaforementioned problem and its object is to provide a servomotor controlapparatus which, by forming a torque command that takes frictionalresistance into consideration, is suitable for application to both asemi-closed loop servo-control system and a fully-closed loopservo-control system.

In accordance with the present invention, there is provided a servomotorcontrol apparatus for controlling movement of a mechanical load by aposition command and a compensating backlash correction command forbacklash at the mechanical load, comprising: position signal detectingmeans for detecting a position of the mechanical load; discriminatingmeans for discriminating, based on the backlash correction command, atiming at which an operating quadrant of the mechanical load of aservomotor reverses; correcting means for correcting, by offset datacorresponding to frictional resistance of the mechanical load, a torquecommand formed from the position signal and the position command whenthe quadrant reverses; and control means for driving and controlling theservomotor by the corrected torque command.

Accordingly, with regard to the fully-closed loop-type servo system, theservomotor control apparatus of the present invention inputs a backlashcorrection command and data indicative of the servomotor operatingquadrant together with an ordinary position command, and outputs atorque command. The torque command corresponds to frictional torque ofthe machine at the time of the quadrant reversal. This torque command isprovided to the servomotor as offset data. As a result, a response delayregarding the position command at the time of the quadrant reversal canbe ameliorated. In addition, a backlash correction command from the sameNC apparatus can be used for control in servo-control systems of boththe semi-closed and fully-closed loop type.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a block diagram illustrating the general arrangement of thepresent invention, in the environment of a semi-closed loop servocontrol system,

FIG. 1(b) is a flowchart illustrating the general arrangement of thepresent invention,

FIG. 1(c) is a block diagram illustrating the general arrangement of thepresent invention in the environment of a fully-closed loop servocontrol system,

FIG. 2 is view illustrating a velocity-torque characteristic,

FIG. 3 is a circuit diagram of a fully-closed loop-type servomotorcontrol apparatus, and

FIGS. 4 and 5 are block diagrams of a servo-control system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will now be described in detailwith reference to the drawings.

FIGS. 1(a) and 1(c) are block diagrams illustrating the generalarrangement of the invention. In the FIG. 1(a), an NC apparatus outputsa position command MC(i) and a backlash correction command BL(i)to asemi-closed loop type servo-control system. The position command MC(i)is and correction command BL(i) are inputted to a comparator B₁, whichcompares the position command MC(i) and a position signal POS(i) to forman error signal E₁. The error signal E₁ is multiplied by the positiongain K and the product is inputted to a comparator B₂ as a velocitycommand E₂. The comparator B₂ compares the velocity command E₂ with theservomotor velocity signal VEL(i) to form an error signal E₃. The errorsignal E₃ is outputted to a comparator B₄ via a comparator B₃,integration term Z⁻¹ and gain k₁, and is operated on by the velocitysignal VEL(i) fed back to the comparator B₄ via gain k₂. The comparatorB₄ outputs the torque command for the servomotor.

A backlash correction flaq BLF(i) formed from the backlash correctioncommand BL(i), as illustrated in FIGS. 1(a) and 1(c) by a', is input toa correction circuit A. If the velocity-torque characteristic of theservomotor is as shown in FIG. 2, the correction circuit A receives thecorrection flag BLF(i) inputted as quadrant data regarding the backlashcorrection command. In there is a reversal in the operating quadrant,which is detected for each axis, of the movable element, time the torquecorrection command FR(i) corresponding to stored frictional torque isoutputted. More specifically, a torque command corresponding tofrictional torque of the machine is applied as offset data, as shown inthe operating flow of FIG. 1(b), in accordance with operating regiondata at the time the direction of rotation reverses.

That is, offset data FR(i) (torque correction command) is formed andinputted to the comparator B₄. The offset data FR(i) is as follows, withreference to FIGS. 1(b) and 2:

    FR(i) =-Fa                                                 (1)

for the third quadrant [BLF(i) <0]; (where Fa represents a presetfrictional torque of the machine) and

    FR(i) =Fa                                                  (2)

for the first quadrant [BLF(i) >0].

In the semi-closed loop-type servo-control system, the backlashcorrection command BL(i) is inputted directly to the comparator B₁ or B₂to correct the position signal. In the fully-closed loop-typeservo-control system, the mechanical load itself is disposed in thepositioning servo loop, as mentioned above. As a result, a backlashcomponent is contained in the position signal, so that the positionsignal itself cannot be corrected. Accordingly, offset data FR(i)(torque correction command) the polarity of which is decided by thebacklash correction flag BLF(i) based on the quadrant data of thevelocity-torque characteristic is inputted by an open loop at theposition at which the torque command in the servo loop is formed, asshown in FIG. 1(a) and FIG. 1(c). The reason for this is that since themove command issued in the servo system usually is substantially zero atthe time of the reversal in the direction of rotation, the polarity ofthe torque command cannot be discriminated from the polarity of the movecommand. Thus, an improvement in response delay is achieved by formingthe torque command at the time of the reversal in direction from thequadrant data regarding the backlash correction command for correctingthe velocity-torque characteristic.

Though an embodiment of the present invention has been described, theinvention is not limited thereto but can be modified in various wayswithout departing from the scope of the claims.

INDUSTRIAL APPLICABILITY

The servomotor control apparatus of the present invention can be appliedto a servo system adapted to perform feedback of a machine position andnot servomotor position in response to a position command.

We claim:
 1. A servomotor control apparatus for controlling movement ofa mechanical load in response to a position command and to a backlashcorrection command for correcting backlash at the mechanical load,comprising:position detecting means for detecting a position of themechanical load; discriminating means for discriminating, based on thebacklash correction command, a time at which an operating quadrant ofthe mechanical load reverses; correcting means for receiving offset datacorresponding to frictional resistance of the mechanical load, and forgenerating a corrected torque command based on the offset data, thedetected position and the position command when the quadrant reverses;and control means for driving and controlling the mechanical load basedon the corrected torque command.
 2. A servomotor control apparatusaccording to claim 1, wherein said position signal detecting meanscomprises an optical linear scale.
 3. A servomotor control apparatusaccording to claim 1, wherein said correcting means comprises means forstoreing the offset data corresponding to a frictional resistance valuefor each axis of the mechanical load.
 4. A servomotor control apparatusaccording to claim 1, wherein said control means comprises either asemi-closed loop servo system or a fully-closed loop servo system, andthe corresponding means comprises a means for generating a correctedtorque command based on the backlash correction command or on the offsetdata.
 5. A servomotor Control apparatus according to claim 1, whereinthe mechanical load is a machine tool or an industrial robot having aplurality of servo systems.
 6. A servomotor control apparatus accordingto claim 1, wherein said discriminating means includes:means fordetermining, based upon the backlash correction command, a time at whicha quadrant in the velocity-torque characteristic for an axis of themechanical load reserves.